Process for selectively treating subterranean formations



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United States Patent Ofiicc 3,115,930 PROCESS FOR SELECTIVELY TREATING SUBTERRANEAN FORMATIONS George G. Bernard, Crystal Lake, 111., asignor to The gtlllre Oil Company, Chicago, 111., a corporation of no No Drawing. Filed Oct. 5, 1959, Ser. No. 844,183

4 Claims. (Cl. 166-33) This invention relates to an improved method for selectively treating the more permeable of a plurality of strata penetrated by a well-bore, and more particularly relates to an improved method for introducing a treating fluid selectively from the well-bore into only the more permeable stratum of the formation.

In oilfield work, it is frequently desirable to treat only one stratum of an oil-containing formation while leaving the formation strata surrounding the remainder of the well-bore untreated. For example, in water-flooding operations, it is often desired to plug selectively the more permeable strata without plugging the less permeable streaks or thief ions, that is, lateral strata which accept water at a higher rate than that at which it is accepted by the remainder of the reservoir, channelling or fingering of the water through the thief section occurs, and uneconomically high water-to-oil ratios are found in the produced fluids before the water front has advanced to a significant extent through the tighter, less permeable strata. Many methods, such as isolation of the thief sec- If the reservoir contains one or more loose tions by means of packers, developing a water-impermeable filter cake on the well face at the thief sections. and selective plugging of the thief sections of the oil-containing formation have been proposed in the past to overcome this difiiculty, but most of these earlier methods have been only partially effective because the water entering the tighter strata has tended to escape vertically into the looser strata after it has advanced only a. very short distance into the reservoir. Moreover, it has not been possible in the past to selectively plug a single, more permeable formation stratum without also plugging, at least to some extent, the tighter strata. Since the tighter strata are already highly sensitive to even moderate decreases in permeability, attempts to plug thief sections have sometimes ended unsuccessfully in the substantial plugging of the entire formation.

In any injection procedure where pressure is the force applied to cause a treating liquid to enter the formation, the natural tendency is for the fluid to enter the formation in proportion to the permeabilities of the various strata. Thus, if a thief section is isolated by means of packers placed in the well-bore above and below the thief agent into the thief section, while the greatest portion of the plugging material will flow laterally outward through the thief streak, a minor proportion of the plugging agent will radiate upward and downward through the less permeable, adjacent strata. Since the less permeable strata are already very tight, the placing of even a minor amount of plugging agent therein can substantially close these adjacent strata.

It is an object of this invention to provide a method for selectively treating the more permeable strata of a subterranean formation penetrated by a well-bore, without also subjecting the adjacent, less permeable strata to the effects of the treating agent.

'It is another object of this invention to provide a method for introducing a treating liquid which is insol-- uble in the interstitial liquids contained in a more permeable stratum of a formation penetrated by a well-bore,

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3,115,930 Patented Dec. 31, 1963 without introducing substantially any of said liquid into adjoining, less permeable strata.

Yet another object of thi'sinvention is to provide a method for introducing a liquid into a subterranean formation penetrated by a well-bore by applying to said liquid a pressure greater than the displacement pressure of the more permeable stratum of said formation, but less than the displacement pressure of the less permeable stratum of said formation.

The method of this invention is based on the injection of selected fluids into a subterranean reservoir at pressures greater than the displacement pressure of the looser streaks, or thief sections, but less than the displacement pressure of the tighter sections. In general, displacement pressure is defined as the lowest pressure at which a given liquid can be caused to enter a capillary or rock pore containing a dissimilar liquid. This pressure is dependent upon the capillary or pore radius, the interfacial tension between the two dissimilar liquids, and other physical factors, and is represented by the equation Table I Capillary Dl :placc mcnt Radius Pressure (microns) (p.s.l.)

Similar relationships exist for other liquid combinations. It is to be understood that considering a capillary having a radius of 1 micron, said capillary initially containing water, a pressure differential of 15 p.s.i. must exist between the internal pressure in said capillary and the pressure of a fluid consisting of hexane which is to be fiowed into the capillary. Thus. it is apparent that higher pressures are required to rupture the interface in smaller capillaries, and conversely, lower pressures are required I to rupture the interface in larger capillaries. This phenomenon is further illustrated by the data Presented in Table II. Tests were conducted using two glass-cores. Both cores were initially saturated with water, and hexane was driven into each core under pressure to replace the interstitial water. The pressure at which the first flow of hexane into the core occurred was measured and is shown in the table.

Table II Core No. Permeability, Initial Flow ss11re,p.s.i.

it is evident thaf. considering a formation containing interstitial water and having a general permeability of 20 md., but having a thief streak with a permeability of 200 injection wells.

md., a hexane solution disposed in a well-bore penetrating the formation can be made to fiow selectively only into the thief streak by maintaining the pressure of the hexane at a value greater than 3 p.s.i. but less than p.s.i. It must be understood that these pressures are differential pressures, that is, the amount by which the pressure of the hexane exceeds the pressure of the interstitial water in the formation pores. It is further to be understood that while the permeability of a formation is in general dependent upon the radius of the capillaries or pores existing in the formation, it is theoretically possible that a formation may contain a thief section which contains an extremely large number of pores which may not be substantially greater in radius than the less numerous pores occurring in the remainder of the formation. Such a thief section would not be susceptible to treatment by the process of this invention. However, it is doubted that troublesome thief sections will ever he found in practice in which the radius of the capillaries in the thief section is not substantially larger than that of the capillaries in the remainder of the formation.

Since the displacement pressure for any capillary system involving displacement of an interstitial liquid by a dissimilar displacing liquid system is proportional to the surface tension existing at the interface between the interstitial and displacing liquids, it is evident that if the interstitial and 'the displacing liquids are miscible, the surface tenson and therefore also the displacement pressure will approach zero. It is therefore necessary to the practice of the method of this invention that the interstitial liquid in the reservoir and the liquid to be introduced into the more permeable streaks be immiscible. and it is desirable that the liquids selected exhibit a high interfacial tension.

The method of this invention is admirably suited to the selective plugging of thief sections surrounding water- Since the reservoir surrounding waterinjection wells contains interstitial water as the predominant pore fluid, the use of formation-plugging reactants comprising hydrocarbon solutions is preferred. To selectively plug such a thief zone, a water-immiscible solution of a chemlcally-polymerizable first reactant is disposed in the well-bore adjacent to the thief zone, and a pressure greater than the displacement pressure of the thief zone to be plugged is applied, but the applied pressure must be less than the displacement pressure of the tighter zones which are not to be plugged. This causes the solution to enter only those streaks which are to be plugged. Then the reactant solution is removed from the well-bore, and an aqueous solution immiscible with the reactant solution, but capable of combining chemically with the first reactant (or catalyzing the reaction of the first reactant) to form a sol-id, insoluble polymer, is introducedinto the well-bore and disposed adjacent to the thief zone. A pressure exceeding the displacement pres- .sures of the thief zone to the aqueous solution is applied to this aqueous solution. Because the first injected solution entered only the more permeable zone which contains. larger capillaries, the two solutions intermingle therein to form an insoluble polymeric material which at least partially plugs only the more permeable and decreases the permeability of this thief zone.

Various materials are known which upon contact will react to form a stratum-plugging polymer. For use in the process of this invention, the first-injected reactant should be dissolved in a water-immiscible solvent, preferably a hydrocarbon. The second reactant (or catalyst) is dissolved in a solvent, preferably aqueous, which is immiscible with the hydrocarbon-reactant solution. Table III lists suitable reactant pairs which may be used with the process of this invention. It is understood that the first reactant will be injected in solution with hydrocar- The first reactant solution should contain an amount of reactant in excess of 10%, and preferably 20-30%. by weight of the hydrocarbon solvent. Sufficient solution should be injected to fill the formation to a radial distance of 3-20 feet from .the well-bore. The second material,'if a reactant. should be injected in stoichiome-trie amount. If a catalyst,0.5 to 5.0% by weight of the first reactant should be injected. The amount of fluid in which the second reactant or catalyst is dissolved is not critical.

In carrying out the process of this invention, it is necessary to control the pressure of the treating liquid disposed in the well-bore adjacent to the more permeable stratum within rather narrow limits. by isolating a section of the well-bore bracketing the zone of higher permeability and maintaining pressure in this zone within the desired limits by means of pressuresensing devices, pumps, and chokes. It is preferred, however, to place a single packer in the well-bore at the lowest extremity of the more permeable stratum. Treating liquid is then introduced into the well-bore and a column of treating liquid formed therein. This column is supported by the single packer and confined by the wellbore walls. The pressure adjacent to the more permeable zone. that is, the pressure adjacent to the lower end of the treating fluid column may be conveniently regulated by controlling the height of the column in the well-bore. As the treating fluid flows into the more permeable stratum, additional fiuid may be introduced into the wellbore to maintain the height of the column of treating liquid at the desired level.

It is desirable to ascertain the displacement pressures of the more permeable stratum liquid system and also the displacement pressures of the formation strata lying above and below the zone to be selectively treated. These displacement pressures may be determined most conveniently by tests conducted on cores obtained from the respec tive strata. If the well is used as a water-injection well. the cores preferably are initially saturated with water by water-flooding the cores to residual oil saturation. A sample of the treating liquid is then disposed adjacent to one surface of the core and the pressure applied to this liquid is gradually increased until fluid-flow into the core begins. The pressure at which flow of fluid into the core begins is measured, and this is the'displacement pressure for that particular core-liquid system. By thus testing a sample core taken from a thief streak to be treated, and also testing core samples obtained from the formation strata lying above and below this streak. at table similar to Table II can be prepared. The critical displacement pressures are the displacement pressure of the thief streak and the displacement pressure of the next-most permeable stratum which is not desired to be plugged. The pressure applied to introduce the treating liquid must be within these critical displacement pressures.

As a specific example of the process of this'invention, an oil-containing formation which is to be produced by water-flood is found to have a general permeability of 20 md. A thief zone lying adjacent to a water-injection well-bore is found to accept water at an abnormally high rate. It is desiredto selectively plug this thief zone. A core taken from the thief zone and found to have a permeability of 200 md., and a core taken from a stratum This may be done Y w a lying adjacent to the thief zone and found to have a permeability of 20 md., are both flooded with water to residual oil saturation. Both cores are then flooded with the later-described hexane solution, and the pressure at which the hexane solution first enters the cores is measured and noted. It is thus determined that the displacement pressure of the water-fiooded ZOO-Ind. permeability core to the hexane solution is 3 p.s.i. It is further determined that the displacement pressure of the water-saturated 20- md. core to hexane is p.s.i. A packer is set in the wellbore just below the thief zone to be treated, and a chemically-polymerizable reactant solution, comprising a hexane solution containing 25% by weight of methyl methacrylate, is disposed in the well-bore above the packer. The height of the column of hexane-methyl methacrylate solution is raised by the flow of additional quantities of solution into the well-bore until a pressure gauge disposed in the well-bore just above the packer indicates apressure head of 8 psi. The column of the hexane-methyl methacrylate solution selectively enters the thief streak at a moderate rate of flow. After it has been determined that 60 cubic feet of treating solution have been injected per foot of thief-zone depth, the remaining quantity of hexane-methyl methacrylate solution is removed from the well-bore and the well-bore is swabbed clean. A second solution, capable of effecting the polymerization of said methyl methacrylate, and comprising a 10% aqueous solutionof hydrogen peroxide, is introduced into the wellbore. The amount of hydrogen peroxide is equal to -l.0% by weight of the methyl methacrylate. Sufficient pressure is applied to cause the aqueous hydrogen peroxide solution to flow into the thief zone and mingle with the previously-injected, hexane-methyl methacrylate, polymerization solution. This treatment is continued until the permeability of the thief streak is reduced to the desired level. Then the remaining aqueous hydrogen peroxide solution is pumped out of the well-bore, the

packer is removed, and the injection of fioodwatcr is resumed.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for selectively plugging the 'more permeable of two adjacent strata penetrated by a well-bore, said more permeable stratum being characterized as having pores of radius larger than the pores in the less permeable stratum, the pores of said strata predominantly containing interstitial water, comprising disposing within said well-bore adjacent to said more permeable stratum a water-insoluble liquid solution of a chemically-polymerizable reactant, applying to said solution a pressure greater than the displacement pressure of the more permeable stnatum, but less than the displacement pressure of the less permeable stratum, to cause said reactant solution to enter only said more permeable stratum, removing from'said well-bore the reactant solution then remaining therein, disposing within said well-bore adjacent to said more permeable stratum an aqueous solution capable of polymerizing said polymerizable reactant on contact with said reactant solution, and applying to said aqueous solution a pressure greater than the displacement pressure of the more permeable stratum, to cause said aqueous solution to enter the more permeahie stratum and produce polymerization of said polymerizable reactant.

2. A method according to claim l in which said reactant solution comprises a hydrocarbon wluu'on of methyl methacrylate, and said aqueous solution contains hydrogen peroxide.

3. A method according to claim 2 in which a packet is disposed in said well-bore adjacent to the lower ex- .tremity of said more permeable stratum to support a. column of said reactant solution in said well-bore, and the pressure applied to that part of the column of reactant solution adjacent to the more permeable stratum is maintained within the limits of said displacement pressures by controlling the height of said column of reactant solution.

4. A method according to claim 3 in which the displacement pressures ot said stratum are determined experimentally by a method comprising obtaining a core from each of said more permeable and less permeable strata, disposing adjacent to one surface of each core a sample of said reactant solution, applying pressures of gradually increasing magnitude to said samples or reactant solution, and measuring the pressures at which said solutions first fiow into said cores.

References Cited in the file of this patent UNITED STATES PATENTS 2,411,793 Kennedy et al Nov. 26, 1946 2,541,688 Cardwell Feb. 13, 1951 2,714,929 Nowak et al. Aug. 9, 1955 2,805,721 Maly Sept. 10, 1957 

1. A METHOD FOR SELECTIVELY PLUGGING THE MORE PERMEABLE OF TWO ADJACENT STRATA PENETRATED BY A WELL-BORE, SAID MORE PERMEABLE STRATUM BEING CHARACTERIZED AS HAVING PORES OF RADIUS LARGER THAN THE PORES IN THE LESS PERMEABLE STRATUM, THE PORES OF SAID STRATRA PREDOMINANTLY CONTAINING INTERSTITAL WATER, COMPRISING DISPOSING WITHIN SAID WELL-BORE ADJACENT TO SAID MORE PERMEABLE STRATUM A WATER-INSOLUBLE LIQUID SOLUTION OF A CHEMICALLY-POLYMERIZABLE REACTANT, APPLYING TO SAID SOLTION A PRESSURE GREATER THAN THE DISPLACEMENT PRESSURE OF THE MORE PERMEABLE STRATUM, BUT LESS THAN THE DISPLACEMENT PRESSURE OF THE LESS PERMEABLE STRATUM, TO CAUSE SAID REACTANT SOLUTION TO ENTER ONLY SAID MORE PERMEABLE STRATUM, REMOVING FROM SAID WELL-BORE THE REACTANT SOLUTION THEN REMAINING THEREIN, DISPOSING WITHIN SAID WELL-BORE ADJACENT TO SAID MORE PERMEABLE STRATUM AN AQUEOUS SOLUTION CAPABLE OF POLYMERIZING SAID POLYMERIZABLE REACTANT ON CONTACT WITH SAID REACTANT SOLUTION, AND APPLYING TO SAID AQUEOUS SOLUTION A PRESSURE GREATER THAN THE DISPLACEMENT PRESSURE OF THE MORE PERMEABLE STRATUM, TO CAUSE SAID AQUEOUS SOLUTION TO ENTER THE MORE PERMEABLE STRATUM AND PRODUCE POLYMRIZATION OF SAID POLYMERIZABLE REACTANT. 